It is known that the polymerization casting of axially symmetrical articles, such as contact lenses, can be performed by using equipment in which individual molds are arranged in a carousel or in a vertical stack configuration. These individual molds, characterized by an outer cylindrical wall and a mold cavity with an exposed concave bottom surface and containing a liquid polymerizable mixture in the cavity, are caused to rotate about their vertical axis at a rotational speed (and under polymerization conditions) sufficient to create a centrifugal force which causes a radially outward displacement of the liquid reactants in the mold. By maintaining the rotating mold(s) under predetermined and known polymerization conditions, the outwardly displaced liquid reactants are caused to polymerize to a solid polymeric contact lens. The resulting lens is characterized by a convex optical surface which corresponds to the concave surface of the mold and a concave optical surface whose geometry has been created, to a significant degree, by the centrifugal force(s) employed during the polymerization cycle.
In the centrifugal casting of contact lenses on a commerical scale, it is preferred for the attainment of good yield to effect the polymerization or curable reaction under an inert gaseous medium such as argon or nitrogen. This is due to the fact that the oxygen component of air entrained within the polymerization column can inhibit the polymerization reaction and adversely affect the quality and acceptability of the contact lens product. A controlled feed of nitrogen through the polymerization column will purge any entrained air in the polymerization zone and provide an inert environment for conducting the polymerization process.
A novel commercial device for centrifugally casting of articles such as lenses is disclosed in copending U.S. application Ser. No. 490,634 filed on May 2, 1983, now U.S. Pat. No. 4,517,138, by the same assignee of the subject application. The disclosure made in this application, to the extent intended herein, is incorporated by reference as if set out in full text. Specifically, the device disclosed in this copending application relates to a device for the centrifugally casting of articles comprising a rotatable polymerization tube (column) adapted for rotation about a vertical (longitudinal) axis and adapted for receiving in vertical series a plurality of molds containing a polymerizable or curable composition; securing means for securing a plurality of molds in vertical series in an interference fitting and sliding relationship within said tube, said securing means being adapted to concentrically dispose said molds to the vertical axis of said polymerization tube; gas flow means associated with the inner surface of said tube and the outer wall of said molds to accommodate a flow of a gaseous medium through said polymerization tube; and said device being operable such that the rotation of said tube causes the synchronized rotation of said molds while maintaining the concentricity of said molds to the vertical axis of said polymerization tube until at least the polymerizable or curable composition in each mold is spin cast into a predetermined shaped article.
Any fluid polymerizable, curable or moldable reactant or mixture with/without an inert or reactive solvent which is/are capable of being displaced outwardly due to the rotation of the column, i.e., by the resultant centrifugal forces, can be employed to fabricate centrifugally cast lenses. The medium comprising the reactant(s) constitutes a homogenous liquid and is capable of forming a desired shaped article during the centrifugal casting operation. The article may be opaque, translucent or transparent depending on the ultimate use of the cast article formed. On the other hand, for example, it is a requirement that soft hydrophilic contact lenses be fully transparent, of good optical quality, soft and resilient as well as possessing other necessary and desirable characteristics.
In particular, a centrifugal casting device coupled to gas feeding means can be utilized in the manufacture of a wide variety of contact lenses which can be symmetrical or asymmetrical; hard, flexible or soft; water absorptive or non-water absorptive; low, medium, or high oxygen permeability or transportability; and the like. By choosing suitably designed mold cavities or bottoms there can be obtained a wide variety of modified lens shapes, e.g., toric, bifocal, truncated and/or ballasted contact lenses. A wide variety of materials or construction can be employed to fabricate the molds; see, for example, U.S. Pat. No. 3,660,545. For the preparation of hydrophilic articles such as soft contact lenses a mold fabricated of a thermoplastic material, such as polypropylene is suitable. To insure proper wetting of the optical surface of the mold by the lens-forming mixture it is desirable to first pretreat or hydrophilize the said surface by known methods.
The liquid lens-forming mixture can comprise monomer, prepolymer or vulcanizable components. Particular suitable components are hydrophilic monomers preferably including those which form slightly or moderately crosslinked, three dimensional networks such as those disclosed in U.S. Pat. No. 3,822,089. Illustrative hydrophilic monomers include water soluble monoesters of an acrylic acid or methacrylic acid with an alcohol having an esterifiable hydroxyl group and at least one additional hydroxyl group such as the mono- and polyalkylene glycol monoesters of methacrylic acid and acrylic acid, e.g., ethylene glycol monomethacrylate, ethylene glycol monoacrylate, diethylene glycol monomethacrylate, diethylene glycol monoacrylate, propylene glycol monomethylate, dipropylene glycol monoacrylate, and the like; the N-alkyl and N,N-dialkyl substituted acrylamides and methacrylamides such as N-methylacrylamide, N,N-dimethylacrylamide, N-methylmethacrylamide, N,N-dimethylmethacrylamide, and the like; N-vinylpyrrolidone; the alkyl substituted N-vinyl pyrrolidones, e.g., methyl substituted N-vinylpyrrolidone; glycidyl methacrylate; glycidyl acrylate; the unsaturated amines; the alkyl ethyl acrylates; solubilized collagen; mixtures thereof; and others known to the art.
Hydrophilic monomers particularly useful in the practice of the invention to manufacture contact lenses include hydrophobic acrylic esters, suitably lower alkyl acrylic esters, preferably wherein the alkyl moiety contains 1-5 carbon atoms, such as methyl acrylate or methacrylate, ethyl acrylate or methacrylate, n-propyl acrylate or methacrylate, isopropyl acrylate or methacrylate, isobutyl acrylate or methacrylate, n-butyl acrylate or methacrylate, or mixtures thereof.
Other suitable monomers include the ethylenically unsaturated monocarboxylic acid esters, in particular, the methacrylic and acrylic acid esters of siloxane monomers and polymers with/without a pendant hydroxyl group. These monomers are well documented in the contact lens art; see, for example, U.S. Pat. Nos. 4,139,548; 4,235,985; 4,152,508; 3,808,178; 4,139,692; 4,248,989; and 4,139,513. The disclosure of the foregoing illustrative patents, to the extent intended herein, are incorporated by reference as if set out in full text.
Among the preferred monomeric mixtures are those which contain at least one alkylene glycol monoester of methacrylic acid, especially ethylene glycol monomethacrylate, and at least one crosslinking monomer such as the alkylene glycol diester of methacrylic acid, especially ethylene glycol dimethacrylate. Such mixtures may contain other polymerizable monomers, desirably in minor amounts such as N-vinylpyrrolidone, methyl methacrylate, acrylamide, glycidyl methacrylate, N-methylacrylamide, diethylene glycol monomethacrylate, and others illustrated above.
The above illustrated monomers, monomeric mixtures including mixtures of hydrophobic and hydrophilic reactants, may be further admixed with a minor proportion of di- or polyfunctional polymerizable species to cause crosslinking of the polymeric matrix as polymerization or curing proceeds. Examples of such di- or polyfunctional species include: divinylbenzene, ethylene glycol diacrylate or methacrylate, propylene glycol diacrylate or methacrylate, and the acrylate or methacrylate esters of the following polyols: triethanolamine, glycerol, pentaerythritol butylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, mannitol, sorbitol and the like. Other crosslinking monomers can be illustrated by N,N-methylene-bisacrylamide or methacrylamide, sulfonated divinylbenzene, and divinylsulfone.
Additional lens-forming materials which are suitable in the fabrication of contact lenses are illustrated by one or more of the following U.S. Pat. Nos. 2,976,576; 3,220,960; 3,937,680; 3,948,871; 3,949,021; 3,983,083; 3,988,274; 4,018,853; 3,875,211; 3,503,942; 3,532,679; 3,621,079; 3,639,524; 3,700,761; 3,721,657; 3,758,448; 3,772,235; 3,786,034; 3,803,093; 3,816,571; 3,940,207; 3,431,046; 3,542,461; 4,055,378; 4,064,086; and 4,062,627.
The polymerization reaction can be carried out in bulk or with an inert solvent. Suitable solvents include water; organic solvents such as water-soluble lower aliphatic monohydric alcohols as well as polyhydric alcohols, e.g., glycol, glycerol, dioxane, etc.; and mixtures thereof. In general, the solvent comprises a minor amount of the reaction medium, i.e., less than about 50 weight percent.
Polymerization of the lens-forming mixture may be carried out with free radical catalysts and/or initiators of the type in common use in vinyl polymerization. Such catalyst species can include the organic peroxides, the alkyl percarbonates, hydrogen peroxides, and inorganic materials such as ammonium, sodium, or potassium persulfate. Polymerization temperatures can vary from about 20.degree. C., and lower, to about 100.degree. C., and higher.
Polymerization of the monomer or prepolymer material can also be effected using, for example, radiation (U.V., X-ray, microwave, or other well-known forms of radiation) with/without the presence of well-known initiator(s) and/or catalyst(s).
The shape of a lens blank may be controlled not only by the size and shape of the mold, but also by the amount and nature of the components comprising the lens-forming mixture.
Although the above-described centrifugal casting process does produce precision lenses, time-consuming steps must be taken to oftentimes remove undesirable flash or the like in order to make the lenses comfortable to the wearer. Generally, the lens is secured on a lathe and then a diamond cutting tool is used to trim a peripheral edge segment of the lens. Since the lens is generally held in the lathe spindle by use of a collet or mandrel and then rotated as the cutting tool is brought into contact with the edge of the lens, the resulting edge profile is dependent upon the profile of the cutting tool and the path the tool takes when contacting the revolving lens. Any deviation in the alignment of the cutting tool and the lens due to misalignment of the mold on the lathe could result in an asymmetrical trim of the peripheral edge segment of the lens. This could result in unacceptable soft contact lenses being produced.